In a conventional structure of an optical fiber, one core with a high refractive index is arranged in the center of a cladding material with a low refractive index. Only in recent years, a so-called multi-core fiber, which contains a plurality of cores with high a refractive index, in a cladding material, increasingly attracts attention, because the multi-core fiber is expected to enable large-volume information transmission and high-speed transmission. The multi-core fiber structure itself is not novel.
As illustrated in FIGS. 34A-34D and FIGS. 35A-35D, in a multi-core fiber 50, a plurality of cores 52 (core diameter: approximately 10 μm) with a high refractive index are arranged in a cladding material 51. As the number of cores contained in the multi-core fiber 50 is larger, the multi-core fiber 50 can transmit a larger volume of information at a time. However, the outer diameter of the multi-core fiber 50 should be about 300 μm at the largest, and taking the diameter of each core 52, a core distance necessary to prevent interference among the cores 52, and other factors into consideration, the number of cores arrangeable in the cladding material 51 is four to seven, or about nineteen, at most.
The distance between the cores in the multi-core fiber 50 is normally set to 40 μm to 50 μm, which is adequate to avoid interference of optical signals passing through neighboring cores, and the outer diameter of the multi-core fiber 50 is set to 120 μm to 180 μm accordingly (in an example illustrated in FIGS. 34A-34D, the outer diameter of the multi-core fiber 50 is 120 μm, and the distance between the cores 52 is 40 μm; and in an example illustrated in FIGS. 35A-35D, the outer diameter of the multi-core fiber 50 is 160 μm, and the distance between the cores 52 is 40 μum). In order to transmit information using the multi-core fiber 50 thus configured, a single-mode fiber 53 is connected to each of the plurality of cores 52 in the multi-core fiber 50. Then, optical information that has passed through each core 52 of the multi-core fiber 50 is received by an optical receiver via the single-mode fiber 53, and an optical signal from each of a plurality of optical transmitters is transmitted to each core 52 of the multi-core fiber 50 via the single-mode fiber 53.